Today, the number of functions provided in a system LSI has been increased and the circuit size of system LSIs tends to be increased. Thus, reduction in the circuit size of system LSIs has been a universal issue.
In almost all LSIs, a phase-locked loop (which will be hereafter referred to as a “PLL”) is provided. As for PLLs, there is the restriction that a maximum value of a response time can not be made to be equal to or larger than about one-tenth of a frequency of an input clock. Therefore, a CR product of a loop filter constituting a PLL has to be set to be relatively large. To achieve a relatively large CR product, in general, a capacitance value of a capacitor constituting a loop filter is set to be large. Accordingly, among components of the PLL, the loop filter occupies a larger portion of a circuit area of the PLL. Since the lower a frequency of an input clock in the PLL is, the larger a response time becomes, the capacitance value of the capacitor constituting the loop filter has to be set even larger. As a result, the circuit size of the PLL is increased. To solve such problems, reduction in the circuit size of loop filters is desired.
As a technique for reducing the circuit size of a loop filter used in a PLL, there is a technique disclosed in Japanese Patent Application No. 2003-185573 by the first inventor of the present application et al. FIG. 16 is a block diagram illustrating a configuration of a loop filter according to the invention (which will be hereafter referred to as “prior application”) disclosed in the specification of Japanese Patent Application No. 2003-185573. The loop filter includes a capacitor 310 (capacitance value C) connected to an input terminal IN1, a resistor 320 (resistance value R) and a capacitor 330 (capacitance value C3) which are connected to an input terminal IN2, and a voltage buffer circuit 350 provided between the capacitor 310 and the resistor 320. Discharge/charge currents (charge currents) Ip1 and Ip2 are supplied to the input terminals IN1 and IN2, respectively, from a dual charge pump circuit. Then, the loop filter outputs a voltage Vout generated at a connection point of the resistor 320 and the capacitor 330. In the loop filter, with a current supplied to the capacitor 310 set to be smaller than a current supplied to the resistor 320, only a capacitance value of the capacitor 310 is reduced without increasing a resistance value of the resistor 320. Thus, the same CR product as that of a known loop filter, i.e., the same filter characteristics as those of the known loop filter can be realized. Moreover, because a sufficient voltage is applied to each of the capacitors 310 and 330, it is possible to realize use of a MOS capacitor as each of the capacitors. Thus, respective sizes of the capacitors 310 and 330 are reduced, so that the entire circuit size of the loop filter is reduced.
The loop filter has to be configured so that a sufficiently large voltage is generated in the resistor 320. Therefore, a current value of a current flowing in the resistor 320 has to be set to be relatively large or a resistance value of the resistor 320 has to be set to be relatively large. However, each of those cases is not preferable because power consumption of the resistor 320 becomes relatively large in either case. Specifically, since the resistor 320 becomes a cause of the generation of noise, it is preferable to avoid increasing the resistance value of the resistor 320.
To reduce noise caused due to a resistor in a loop filter, a loop filter including, instead of a resistor, a switched capacitor circuit has been well known (see, for example, Patent Reference 1). FIG. 17 is a block diagram illustrating a configuration of a known loop filter including a switched capacitor circuit. The loop filter includes capacitors 310 (capacitance value C) and 330 (capacitance value C3) connected to an input terminal IN and a switched capacitor circuit 320A connected to the capacitor 310. The loop filter outputs a voltage Vout generated at a connection point of the capacitors 310 and 330. The switched capacitor circuit 320A includes a capacitor 340 (capacitance value CR) and switches Q1 and Q2 for switching a connection direction of the capacitor 340. The switched capacitor circuit 320A substantially exhibits a resistance value R. In the above-described configuration, a resistor is omitted from a loop filter, so that noise caused due to the resistor can be reduced.    Patent Reference 1: U.S. Pat. No. 6,420,917, specification, (pp. 6-7, FIG. 4).